1. Field of the Invention
The present invention relates to the field of fabrication of semiconductor transistors. Specifically, the present invention relates to the formation of MOS devices.
2. Prior Art
Metal-Oxide-Silicon, MOS, transistors such as Field Effect Transistors, MOSFETs, are well known in the art. Such devices are typically formed having a source region and a drain region, of similar conductivity type, separated by a channel region, of a differing conductivity type, capped with a conductive gate. The gate to source voltage controls the passage of current through the channel between the source and the drain regions. In typical n-channel operation, a positive voltage is applied between the drain and the source with the source grounded to a reference potential. Due to the differing conductivity type of the channel separating the source and the drain, usually no current flows between the source and drain. However, if a sufficiently large voltage is applied between the gate and source, the conductivity in the channel region will increase, thereby allowing current to flow between the source and the drain. The gate voltage required to induce the flow of current between the drain and the source is referred to as the threshold voltage.
Under certain circumstances, however, unwanted current flow may occur between the source and the drain even when no voltage is applied to the gate. Such a condition may be due to avalanche breakdown or punchthrough. Punchthrough occurs when the MOS transistor is biased in an off state with the gate and the source both at approximately zero volts with respect to ground, but with the drain at a voltage as high as 5 volts. Even though no flow of current is desired, drain current may still occur regardless of the zero gate voltage. This is due to the fact that under such conditions, the normal doping concentration of the channel region is not sufficient to prevent current flow between the source and drain regions.
In order to eliminate punchthrough currents, the doping concentration in the substrate of the MOS device is raised. A high energy or so-called "punchthrough" implant is used to locally raise the doping concentration of the MOS device substrate. Typically, the "punchthrough" implant is made as a blanket implant over the active region of the MOS device. Unfortunately, the punchthrough implant also raises the doping concentration of the substrate in the source and drain region. As a consequence of the increased doping concentration, the source-drain junction capacitance is also increased.
Furthermore, MOS semiconductor transistors, such as MOSFETs, often experience current leakage and other problems due to short channel lengths. The short channel, which occurs as a consequence of difficult to control manufacturing processes, results in closely spaced source and drain regions. Due to the close proximity of the source to the drain, current leakage or other "short channel" effects may hamper the performance of the semiconductor device.
Consequently, a need exists to prevent punchthrough effects in semiconductor devices such as MOSFETs using a high energy or punchthrough implant without substantially increasing source-drain junction capacitance, and which minimizes short channels effects.